Traditional techniques estimate temperatures in exhaust systems of operating engines on the basis of the operating state of the engines. For example, a typical technique estimates the temperature of a catalyst disposed in the exhaust system of the engine and the temperature of exhaust gas flowing into the catalyst on the basis of the amount of heat generated in the engine. Unfortunately, this technique is difficult to adopt for estimation of temperatures in an exhaust system of a nonoperating engine because the stopping engine generates no heat. In order to solve this problem, an improved technique estimates temperatures in an exhaust system of a stopping engine in view of the balance of heat traveling in the catalyst (for example, refer to International Publication No. WO 2010/013365 A1).
Unfortunately, the amount of heat traveling in the catalyst varies depending on the shape of the catalyst, the composition of a catalytic element, the type of a carrier, the internal structure, and the amount of heat generated by catalytic reaction of components of exhaust gas. It is thus difficult to specify a calorimetric model for accurate temperature estimation, leading to low accuracy of estimating temperatures in the exhaust system. In addition, the temperature of the catalyst varies depending on the environmental temperature (ambient temperature) and the amount of heat remaining in the engine or the exhaust system connected to the catalyst. Thus, the information on the balance of heat traveling in the catalyst alone is insufficient for accurate estimation of the temperature of the catalyst.
The temperatures can be directly detected with two or more thermosensors disposed upstream and downstream of the catalyst. Unfortunately, the apparatus can include the limited number of thermosensors since an increase in the number raises the costs. Installing a great number of thermosensors in an exhaust path may lead to insufficient accommodation for the thermosensors.